1. Field of the Invention
The invention relates to a finger tester probe.
2. Description of Prior Art
Circuit board testers may be basically classified in two groups, namely finger testers and parallel testers. Parallel testers are test devices simultaneously contacting all or at least the majority of the contact points of a circuit board being tested by means of an adapter. Finger testers are test devices for testing non-componented or componented circuit boards which sequentially sample the individual contact points with two or more test fingers.
As a rule the test fingers are secured to a slide travelling on crossbars which in turn are guidingly travelled on guide rails. The slides can thus be positioned to any location of a test area which as a rule is rectangular. For contacting a contact point on the circuit board under test the slide is vertically shiftable on the crossbar so that the test finger is landable on the contact point of the circuit board from above or below.
A finger tester is described in EP 0 468 153 A1 and a method for testing circuit boards by means of a finger tester is described in EP 0 853 242 A1.
Known from U.S. Pat. No. 5,113,133 is a test probe in which a leaf spring serving as the probe element is guided in a linear cavity. For contacting the surface of a test object the probe element is shiftable downwards in this linear cavity, it thereby extending with a tip from the linear cavity to contact a contact pad of the test object. By the end of the contact spring opposite the tip of the leaf spring the contact spring is secured to a rotatable disc, rotation of which causes the leaf spring to be extended or retracted, thus enabling the leaf spring to be moved up and down in the linear cavity by rotation of the disc.
The linear cavity is provided with a side cavity in which the leaf spring is able to relax so that the reactive force occuring in xe2x80x9clandingxe2x80x9d on the surface of the test object is absorbed.
This probe is intended to provide fast contact between the probe element and a contact pad of the test object whilst the impact of the probe element on the surface of the test object is designed to be non-injurious thereto.
U.S. Pat. No. 5,804,982 describes a test probe for a microchip tester. The test probe comprises an actuator for vertically displacing a probe tip. The actuator consists of two solenoids arranged horizontally and two magnets arranged to the side thereof. This actuator moves two elastic lever arms fixedly arranged at one end, to the other ends of which a mounting element is secured on which in turn the contact element is arranged. In this known test probe the probe tip is moved together with the mounting element against the spring force of the elastic lever arms.
The criterion substantial to finger testers being a success in the marketplace is the speed with which the contact points of the test object can be contacted. Conventional finger testers require approximately 120 ms for a vertical movement of the test finger. The test probe known from U.S. Pat. No. 5,113,133 is faster and requires only a few 10 ms for a vertical movement.
The invention is based on the object of providing a finger tester probe which is of a simple configuration and is capable of moving a probe element at high speed for contacting a contact point of a test item.
This object is achieved by a finger tester probe comprising a probe element for electrically contacting a contact point of a circuit board under test, an actuator including a permanently premagnetized core and a contact element each movable relative to the other, whereby one of the two parts is fixed and the other part is movable, the movable part being mechanically connected to a probe element so that on being energized the movable part is moved together with the probe element, and the movable part being arranged freely movable relative to the fixed part.
In accordance with the invention the test probe comprises an actuator including a permanently premagnetized core and a contact element movable relative to each other, whereby one of the two parts is fixed and the other is movable, and the movable part is mechanically connected to a probe element so that the movable part is moved together with the probe element on being energized.
The invention is characterized by the movable part being arranged freely movable relative to the fixed part, i.e. no mechanical connection existing between the movable part and the fixed part. Merely one electrical conductor for electrically connecting the probe element to a tester is provided between the fixed and movable part, this electrical conductor being configured so that it is capable of transmitting no or only negligable forces between the movable part and fixed part, as a result of which the movable part is quickly movable without needing to overcome a mechanically resistance.
In one preferred embodiment of the invention the permanently premagnetized core is fixed and the solenoid element forms the movable part which is mechanically connected to the probe element. Since in accordance with the invention the solenoid element and not the core is moved, the mass of the moving parts and thus the inertia of the actuator is maintained low, as a result of which an exceptionally fast movement of the probe is achieved.
In addition to this when the actuator is activated, the solenoid element assumes in each case a zero position and a contact position by predetermining the polarity of the energizing current. Changing over the polarity is abrupt, the being the reasons why also the solenoid element is activated correspondingly abruptly. In addition, for maintaining the solenoid element in the zero position no spring element or the like is needed as is provided in conventional solenoid-operated actuators. Such spring elements increase the inertia of the actuator, this being the reason why maintaining the zero position by means of a holding current enhances the response.
One particularly advantageous embodiment of the test probe is one in which the probe element is an elastic wire guided in a guide passage provided with several cavities. Experience has shown that with such a test probe the force exerted by the wire on a contact point initially increases proportional to the actuating travel and hardly increases further as of a predetermined actuating travel. It is this effect that limits the maximum force exerted on the circuit board.
This effect results from the following:
For curving the wire in a first cavity a predetermined force is needed which is proportional to the actuating travel of the wire as shifted by the actuator and is dicated by the stiffness of the wire and the geometry of the cavity. If the wire is curved in a first cavity, it is curved in a second or third cavity on further actuation by the actuator, whereby the force to be applied by the actuator in this case or the counterforce to be provided by the contact point corresponds to the force needed in the first cavity. The further, very slight increase in the forces is caused by the additional friction between the wire and the guide passage. By this configuration, of the test probe the force exerted maximally by the probe element on the contact point of a circuit board is limited to a predetermined value. Limiting the force in this way follows constructively, this being the reason why there is no need to limit the feed rate when activating the probe element for contacting the contact point as is known in conventional finger testers.
This principle of constructively limiting the force exerted by the probe element on the circuit board under test is a gist of the invention.
A further embodiment includes a finger tester comprising a means for adjusting the vertical position of the test probe and the test probe is provided with an actuator capable of actuating a probe element independently of the means for adjusting the vertical position of the test probe in the vertical direction for contacting a contact point of a circuit board.
By the means for adjusting the vertical position of the test probe a coarse setting of the test probe relative to a circuit board under test is undertaken whereby the spacing between the circuit board and the test probe is adjusted preferably to a small value such as e.g. smaller than 2 mm or smaller than 1 mm. As a result of this, the actuator needs to actuate the probe element only by this short spacing between the circuit board and the test probe which is much simpler and quicker to implement than when the usual spacing of a few millimeters between a starting position of a test finger and a circuit board under test needs to be overcome.
It is this high-speed contacting of a contact point that makes the test probe in accordance with the invention particularly suitable for testing non-componented circuit boards requiring a very large number of circuit board test points to be tested. The test probe in accordance with the invention is also suitable for testing componented circuit boards, however.